1. Field of the Invention
The present invention relates generally to the field of electronics, and more specifically to a method and an associated apparatus for measuring and diagnosing faulted insulated gate bipolar transistors (“IGBTs”).
2. Description of the Related Art
The bipolar junction transistor (“BJT”), including its extension, the Darlington device, and the metal oxide semiconductor field effect transistor (“MOSFET”) are commercially-available advanced electronics devices. Each device has characteristics that complement the other in some respects. Relative to MOSFETS, BJTs have lower conduction losses in the ON-state and larger blocking voltages, but also have lower switching speeds. In contrast, MOSFETs switch relatively faster, but have relatively larger conduction losses in the ON-state. In order to overcome these performance limitations of the BJT and the MOSFET, the IGBT was designed. This device has significantly superior characteristics for low and medium-frequency applications in comparison to the BJT and the MOSFET Specifically, the IGBT is a voltage control device that can turn ON and OFF at a very high speed, and can deliver very high current compared to conventional bipolar transistors. Furthermore, its power rating can be improved by increasing both current and voltage. For this reason, IGBTs are preferred in some applications over both BJTs and MOSFETs.
IGBTs presently serve a number of traditional markets, including motor drives and welding applications. However, with the emergence of new market segments, it is expected that IGBTs will continue to be a growing part of other industries, such as the semiconductor industry. In particular, the automotive and power supply markets, including uninterruptible power supplies (“UPSs”) and switch mode power supplies (“SPSs”), are expected to drive near term growth.
Due to the cost reduction and performance enhancement of the microprocessor, three-phase AC motor drives are becoming increasingly popular and may eventually replace conventional DC motor drives as the dominant motor drive. Presently, in the electric vehicle (“EV”) field, almost all EVs, including hybrid electric vehicles (“HEVs”) and fuel cell vehicles, use AC motor drives. One of the most important functions of the AC motor controller is to convert DC power to three-phase AC power. IGBTs are typically utilized to perform this conversion.
Referring to FIG. 1, there is illustrated the structure of a typical IGBT 1. This structure is very similar to that of a vertically-diffused MOSFET, featuring a double diffusion of a p-type region and an n-type region. An inversion layer may be formed under a gate 2 of the IGBT 1 by applying the correct voltage to the gate contact 3, much like a MOSFET. The main difference between the MOSFET and the IGBT is the use of a p+ substrate layer in the IGBT for a drain. Because of this, the IGBT 1 becomes a bipolar device as the p-type region injects holes into the n-type region.
The gate voltage, VG, controls the ON/OFF state of the IGBT 1. If the voltage applied to the gate contact 3 with respect to the emitter 4 is less than a threshold voltage, VTh, then no MOSFET inversion layer is created and the device is turned OFF. In this instance, any applied forward voltage will fall across a reversed bias junction, J2. The only current to flow will be a small leakage current.
To turn ON the IGBT 1, the gate voltage VG is increased to a point where it is greater than the threshold voltage VTh. This results in an inversion layer forming under the gate 2, thereby providing a channel linking the source to the drift region of the IGBT 1. Electrons are then injected from the source into the drift region, while at the same time junction J3, which is forward biased, injects holes into the n− doped drift region. Some of the injected holes will recombine in the drift region, while others will cross the drift region via diffusion and will reach the junction J3 with the p-type region where they will be collected.
The p-type region exhibits a type of lateral resistance. If current flowing through this resistance is high enough, it will produce a voltage drop that will forward bias the junction with the n+ region, turning ON a parasitic transistor that forms part of a parasitic thyristor. Once this happens, there is a high injection of electrons from the n+ region into the p-type region, resulting in loss of all gate control. This is known as latch up, and usually leads to device destruction.
FIG. 2 is a circuit diagram illustrating a test structure 10 for performing diagnostics on IGBTs in a manufacturing facility. Six IGBTs, individually referenced as A+, A−, B+, B−, C+ and C−, are provided electrically coupled and drivable as a three-phase AC inverter 12, which is to be tested. The test structure 10 includes a voltage source Vdc, a tester 14, an inverter drive 16 including a microprocessor (not shown in FIG. 2), a controlled area network (“CAN”) 18, and a test circuit 20. The tester 14, located at the end of the manufacturing line, is coupled to the microprocessor of the inverter drive 16 via the CAN 18 for passing commands and data between the tester 14 and the microprocessor. The test circuit 20 includes five relays, individually referenced as Ra1, Ra2, Ra3, Ra4 and Ra5, a current sensor 22 and a current limiter 24. The tester 14 controls the relays Ra1, Ra2, Ra3, Ra4 and Ra5, in the test circuit 20, monitors the voltage of each of the IGBTs A+, A−, B+, B−, C+, C−, and makes decisions based upon production tests. The inverter drive 16 provides drive signals to the gates of the IGBTs A+, A−, B+, B−, C+, C−, which are synchronized with the states of the relays Ra1, Ra2, Ra3, Ra4 and Ra5 by the tester 14 to selectively turn each individual IGBT ON and OFF, one at a time, thereby controlling the current going through the IGBTs during testing. Two current sensing signals Ia and Ib provide the phase current to the microprocessor of the inverter drive 16. The collector-emitter voltage Vce across the collector c and emitter e of each of the IGBTs, is measured during the testing of the IGBTs and provided to the tester 14.
Testing of the IGBT switching circuits, for example, A+, requires control relays Ra1 and Ra5 to be closed. After the microprocessor of the inverter drive 16 commands IGBT A+ ON, current will travel through its collector and emitter terminals, c and e respectively, relay Ra1, current limiter 24 and relay Ra5. If IGBT A+ is not faulty, the collector-emitter voltage Vce across the IGBT A+ will be close to zero volts and the current feedback Ia will be equal to a predetermined value. The microprocessor of the inverter drive 16 reads the phase current Ia and sends that value to the tester 14 via the CAN 18.
Further reference to FIG. 2 shows the absence of a current feedback sensor for the C phase, and hence, no current information available for the C phase. This is due to hardware limitations and cost. Accordingly, the two C phase IGBTs C+, C− can only be tested with a measurement of the respective collector-emitter voltages Vce. In order to test all six IGBTs A+, A−, B+, B−, C+, C−, at least six measurements are required. Among the six measurements, four require both current and voltage measurements, while two require only voltage measurements.
As illustrated above, the IGBTs have rather complicated gate drive circuits and can be easily damaged resulting in undiscovered errors. This makes the manufacture of IGBT based power circuits and drive circuits difficult and complex. Further, in the case of a faulted IGBT 1 in the field, diagnostics that pinpoint the exact faulted transistor are also difficult and challenging. Accordingly, there is a need for an improved method of detecting faulted IGBTs.